1. Field of the Invention
The present invention relates to data serialization in general, and to pre-emphasis in high-speed data serialization applications in particular.
2. Related Art
High-speed data serialization is an important technique for transmitting data from a transmitter to a receiver. Such techniques are commonly used in portable computing devices, when data needs to be transferred from a video card to a monitor or other display. Other applications of similar technologies can be found in cellular base stations, where high-speed data serialization is used to pass data between various subsystems. The underlying process for high-speed data serialization is relatively straightforward: a number of slower signals are combined to make a single, faster signal; the same amount of data is transferred in the same amount of time, but far fewer separate signals must be maintained. An example would be the combination of ten separate 10-Megabit signals into a single 1-Gigabit signal.
When data is serialized and transmitted over a medium, such as a cable, the signal is subject to degradation. In differential signals, this manifests as the loss of “transition points”, places where the waveforms cross. See FIG. 1A. Differential signal 101 has transition points as designated by arrows 111 and 112. This degradation is most common in the cases of long 1's in conjunction with short 0's (and long 0's in conjunction with short 1's). In such situations, the transfer medium, e.g. the cable, reaches a state of high (or low) voltage, and the short change in state is not long enough to allow the medium to fully discharge. See FIG. 1B. Differential signal 121 has changes in state at arrows 131 and 132, but the change is too short to allow the medium to discharge; no transition point occurs.
Signal degradation is a well-known problem in the field, and the commonly adopted solution is to use pre-emphasis. Pre-emphasis involves detecting transition points and applying additional current at the detected points. Additional current is provided only at transition points, as most applications that require pre-emphasis are also sensitive to power consumption, e.g. notebook computers. Two approaches are commonly used for pre-emphasis. In both cases, detecting transition points involves comparing the serialized signal with a slightly delayed version of the same stream. Where transition points occur, the signal and the delayed signal will differ. See FIG. 1C. Signal 141 and delayed signal 151 are identical, except that delayed signal 151 is one bit-width behind signal 141. Transition points are detected at times where the signals do not match, as indicated by arrows 161, 162, 163, and 164. It should also be noted that signal 141 has a “long 1, short 0” at the time interval indicated by arrow 162.
In the first approach to pre-emphasis, serialized data enters the circuit and is duplicated. One copy of the signal flows directly to an XOR gate, while the other flows first into a series of fixed delays and then into the same XOR gate. In the XOR gate, the two signals are compared; when a difference is detected, a transition point has occurred and additional current should be applied. The fundamental weakness to this approach is in the nature of the fixed delays. Bit-width, i.e. the time between receipt of one bit of information and the receipt of the next, is not a constant: it obviously varies with frequency. Also, the fixed delays built into circuits of this type are not always constant; the delays change with variations in process, voltage, and temperature (PVT); a 1 nanosecond delay could become a 2 ns delay, or 0.5 ns. As such, circuits embodying this approach may well be applying pre-emphasizing current where it is not called for, or not applying current where it should.
The second approach handles frequency and PVT variations far better. During the serialization process, two serialized signals are created, one skewed one exactly one bit behind the other. These two signals will always be precisely one bit-width apart, and can be compared by an XOR gate to determine where pre-emphasis should be applied. The drawbacks to this approach are not related to its effectiveness, but rather to its desirability. The circuits involved in creating two separate serialized signals are more expensive and far more demanding in terms of energy than duplicating a single signal and delaying it. This is of crucial importance in the portable computing market, where efficient power consumption is vital.
At present, no single approach to pre-emphasis in data serialization provides a low-power solution to the signal degradation problem that allows for variation in frequency, or fluctuation in delays caused by PVT factors.